As the integrated circuit industry has explored ways of packing more circuits onto a given semiconductor substrate, more and more thought has been devoted to not only orienting the various devices in a planar fashion along the surface of the substrate, but also to orienting the devices vertically either by building the devices up from the substrate surface or by burying the devices in trenches formed within the face of the semiconductor body.
For example, a number of schemes for isolating customary planar devices with trench isolation regions are seen in U.S. Pat. No. 4,549,927 to Goth, and the articles "A Bipolar Process That's Repelling CMOS," Electronics, Dec. 23, 1986, pp. 45-47 and "Slot Isolation Yields Densest Bipolar PROM Yet," Electronic, Feb. 10, 1986, pp. 35-37. Trench capacitors are being employed on a commercial basis on the largest memory chips, as described by T. Costlow in "Trench Capacitors Trim Die Size of 1-Mbit DRAM," Electronic Design, Feb. 6, 1986, p. 34. Trench-like transistors are seen in U.S. Pat. No. 4,546,535 to Shepard, and U.S. Pat. Nos. 4,571,512 and 4,571,513, both to Schutten, et al. Another trench type transistor of vertical configuration is seen in U.S. Pat. No. 4,566,172 to Bencuya, et al.
From the above brief history, it is noted that more and more complex devices are being designed for trench manufacture. The next challenge for trench technology naturally involves placing more than one device in the same trench, such as a transistor and a capacitor to form a dynamic random access memory (DRAM) memory cell. Such an approach has been reported by B. C. Cole in "IEDM: How They're Going to Build the 4-Mb DRAM," Electronics, Dec. 2, 1985, where a proposal was described for placing a trench transistor on top of a trench capacitor together in a very deep, shallow trench. One odd feature of the cell is that the trench transistor is hollow, which essentially wastes the central portion of the trench. See also "DRAMs Advance to 4-Mb Level," Electronics, Feb. 17, 1986, pp. 26-27.
Nevertheless, as trench technology is still developing, a need exists for additional trench structures, particularly of the newer, multiple device type, so that the best alternatives may be made available and considered in the fabrication of extremely high density integrated circuits.